how can i fix my radio for my nissan pintara?

everytime i drive along and turn my radio on it cuts out after 5 minutes and says input code the problem is i was never given a code when i bought the car so i dont kno what i am supposed to put in let alone how to put it in its a 1992 nissan pintara 4 door if u need nemore details i would only be abliged to pass on more information the radio is standard for nisssans it doesnt have a name on the outside so i dont kno the name of the radio